Economic viability of phase-changing materials in residential buildings – A case study in Alice Springs, Australia
Introduction
The rapid growth of the global human population, continued economic development, and high living standards impose an increasing energy demand, representing a serious threat to Earth's climate and sustainability. By 2010, the building sector accounted for 32% of global final energy demand (24% residential and 8% commercial) and 30% for energy-related greenhouse gases [37]. These values increased by 4% and 9% in 2018 compared to 2010, respectively [18]. The latest report by the International Energy Agency shows that the overall residential sector accounted for over one-third of the global final energy consumption and became responsible for 40% of CO2 emissions (total and indirect) in 2019 [22], [23]. The energy consumption in the residential sector increased further due to the Covid-19 pandemic when lockdown prompted the population to use more high-energy demand devices at home. For example, during the pandemic, the residential energy demand in the US increased by 6–8% compared with statistics in 2019 for the same period (IEA, 2020b).
The Australian residential sector is not very different. From 2013 to 2014, it accounted for approximately 11% of the country's final energy consumption, with two-thirds of this attributed to space cooling or heating and water heating [20]. Between 1979 and 2010, Australia's total energy usage increased by 90%, with residential buildings alone consuming 11% of the energy and contributing to 13% of Australia's national greenhouse gas emission inventory [2]. Given that 95% of Australia's total energy comes from fossil fuels and energy consumption associated with residential buildings is likely to remain high, energy efficiency measures become a need of the hour [2].
The tremendous potential to use energy-saving measures in the residential sector as an effective mechanism to tackle climate change is globally acknowledged [20], [17]. The concept of active insulation addresses this problem by decreasing the heating and cooling energy demand in buildings without high energy-consuming devices [25]. The idea is to benefit from the insulation layer built in the walls. The optimal use of thermal insulation has been a standard strategy adopted to reduce heating and cooling energy losses [1]. Most thermal insulators can be classified into the following types: vegetal (e.g., cork, wood fiber, hemp fiber, and coconut fiber), animal (e.g., sheep's wool), mineral (e.g., glass fiber, mineral wool, expanded vermiculite, and aerogel) or synthetic insulators (e.g., polyurethane, polystyrene, and polyethylene foam) [7]. Amongst these categories, composite materials using glass fiber, epoxy, and phase-changing materials (PCM) have received particular attention in the building sector and the scientific community for their potential involvement in the efficient usage of energy [1], [38], [2]. Phase-changing materials require integration into the wall structures, which comprises another insulation layer. Thus the right combination of the wall structure with PCM can result in better energy savings than the other insulation materials [28], [14].
Phase-changing materials absorb and release energy proportional to their latent heat of fusion when their temperature goes below or exceeds the phase-changing temperature. Therefore, PCMs can improve energy consumption by absorbing, storing, and releasing useful energy. Numerous material mixtures can be used to create PCM, including both organic and inorganic materials. In construction, they can be used as a separate layer of material, or micro-encapsulated into another, usually insulation, material. The choice of PCMs mainly depends on the required operating conditions, comfort level, and financial feasibility [16]. In their work, Floros et al. focused on organic PCM based on fatty acids as a functional material; however, their conclusions are not limited to any particular PCM type and apply to inorganic PCMs.
Solgi et al. [34] numerically studied the dependency of PCM wall efficiency on different parameters. These parameters included PCM layer thickness, PCM transition temperatures, insulation envelope R-values, and building orientation. Solgi et al.’s work was validated with a full-scale calorimeter and considered the PCM type based on the availability of data and research on its behavior and fitness for the Australian climate. Solgi et al. performed simulations with parameter variations according to four different climate conditions in Australia. The results of their study revealed the optimum values for each parameter to obtain the maximum reduction of energy consumption in buildings of four cities in Australia. It was found that an increased PCM layer thickness for hot-dry and subtropical climates resulted in a significant decrease in cooling load demand, which, as expected, is also benefited with the north orientation of the PCM-layered walls in the Southern Hemisphere. Furthermore, a fixed thin PCM layer effectively saved energy through thicker insulation with a more significant R-value.
In contrast, Solgi et al. found that a well-insulated layer without PCM is more energy-efficient than a combination of insulation with PCM in tropical climate zones. For cold climate zones, Solgi et al. found that the larger the PCM layer thickness, the higher the optimal transition temperature of the PCM. They also reported that the thicker insulation increases the efficiency of the PCM layer when it faces any direction other than the east. Their study results gave PCM wall parameters, which resulted in energy-efficient buildings in different climate zones of Australia. Thus, the study by Solgi et al. [34] was taken as a reference for the present investigation. Their results are used for further examination of the economic viability of energy-efficient buildings in Australia.
Literature review
The PCM is a proven technology that can improve space heating–cooling energy consumption in buildings by absorbing, storing, and releasing useful energy. Early studies have shown the significant benefits of PCM integration in walls regarding thermal energy storage, indoor temperature, and energy consumption variations [15]. Feldman et al. [15] showed that the thermal energy storage of gypsum wallboard could be increased by ten times with the addition of a PCM. Hunger et al. [21] reported energy savings of up to 12% by introducing 5% microencapsulated PCM in self-compacting concrete. Furthermore, Shilei, Neng and Guohui [33] observed a decrease in room temperature by 1.02 °C in Northern China in the winter season by adding PCM into wallboard. A similar study in Northern China by Chen et al. [8] showed up to 17% of energy savings when the phase-changing temperature was set to 23 °C and enthalpy was set to 60 kJ/kg during winter. Additionally, Athienitis et al. [4] investigated the benefits of PCM in passive solar buildings in terms of energy savings and reducing room overheating, and observed a decrease of 4 °C in maximum room temperature in Montreal, Canada, in winter, when using gypsum board coupled with 25% butyl stearate PCM.
A review by Kyriaki et al. [27] remarked on the limited previous work studying PCM as insulation material from a financial point of view. Among the few, Panayiotou, Kalogirou and Tassou [31] considered the use of a PCM layer coupled with insulation in the Mediterranean climate of Cyprus and concluded that PCM is only financially attractive in case of use in conjunction with additional thermal insulation plaster. Some authors of studies conducted after 2017 proposed using PCM as indoor decoration in apartment buildings [39] and as part of a photovoltaic thermal system with liquid circulation [3]. Both studies claim economic viability; however, the former is more a modernization case with no construction involved. The latter is much more of a complex system than the addition of a plain PCM layer. Souayfane et al. [35] conducted experimental research of PCM layer coupled with insulation in different climate zones, concluding that climate is a significant factor in financial attractiveness. Their study covers Mediterranean (Csa), Oceanic (Cfb), Humid Continental (Dfa), Continental (Dsb), Continental Subarctic (Dfc), and Polar (ET) climate zones. Souayfane et al. concluded that PCM layer inclusion is most financially viable in ET and Dfc climates.
Hot Desert (BWh) climate analysis is not well-covered in the literature, and this is very relevant to Australia since most of its territory is desert. Generally, the broad portfolio of climatic conditions present in Australia makes choosing the properties of a PCM for building insulation a significant challenge. Additionally, the use of PCM as an energy efficiency measure in the Australian residential sector has not been thoroughly investigated to determine a range of operating temperatures and PCM types for the country's different climate zones [34]. Nevertheless, Solgi et al. [34] studied a PCM layer coupled with polystyrene thermal insulation for different Australian climatic zones. Solgi et al. also examined the relationship between PCM layer thickness, transition temperatures, envelope R-values, and building orientation in four cities within different climate zones. They used a test wall containing different PCM and insulation thicknesses to calibrate a thermal model simulated in the EnergyPlus2 platform [34].
The Energy Consumer Sentiment Survey, a questionnaire prepared by Energy Consumers Australia [13] and applied to 2,000 households, revealed that only 26% of the tenants consider their home energy efficient. If this nationwide panorama can be interpolated to the 11,000 dwellings in Alice Springs [6], a significant improvement opportunity for energy efficiency measurements, such as PCM, remains untapped. Therefore, there is an evident gap in the need to investigate the economic viability of PCM coupled insulation in the Australian climate for residential buildings. Hence, this work addresses the financial viability of the PCM layer coupled with insulation under the Australian hot arid climate for small residential buildings.
Section snippets
Methodology
This study aims to use the numerical results of Solgi et al. [34] to determine the financial viability and greenhouse gas emission reduction of implementing the PCM layer directly on top of a conventional insulation layer from inside the house on a small residential building in Australia. Our work uses data given for Alice Springs, a city with a hot-arid (BWh) climate. The techno-economic analysis is performed using the RETScreen Expert platform [30] to model a 200 m2 proxy-house (18.33 m-long,
Case Study: PCM implementation to non-compliant envelope
Among the different envelopes modeled by Solgi et al. [34], only the 120 mm and 160 mm polystyrene equivalent envelopes with respective thermal resistances of 3.29 m2·°C/W and 4.29 m2·°C/W meet or exceed the energy efficiency measures stated by the Australian building code. From those two, the thinner envelope of 120 mm is analyzed.
According to the results obtained by Solgi et al. [34], thinner envelopes, such as the 80 mm insulation equivalent (R = 2.15 m2·°C/W), if coupled with a 30 mm thick
Discussion
The proposed building envelope insulation, including a PCM layer, is financially viable in the hot-arid climate of Australia in case of the introduction of a Grant or the use of low-cost PCM technology. The significant reduction in GHG emissions brought with this insulation scheme compared to traditional single-layered insulation makes this technology attractive from an environmental standpoint.
Regarding the possibility of Grants, in 2010, the Commonwealth of Australia implemented the National
Concluding remarks
The techno-economic viability of using a PCM in building insulation in the region of Alice Springs, Australia, is presented in this study. The study uses a life-cycle cost analysis approach and a proxy heat pump to replicate the energy storage-release features of PCM coupled insulation in a year-round calculation. Results can be summarized as follows:
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Despite the transient complexity of modeling latent heat thermal storage like PCMs, a financial decision-support tool like RETScreen Expert can
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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